The present invention generally relates to aircraft engine fuel control systems, and more particularly is concerned with automatic throttle and synchronization systems for multi-engine aircraft.
In recent years, the cockpits of modern multi-engine aircraft have become increasingly crowded with complex avionics equipment, which provides the flight crew with much-needed information, while concomitantly requiring increased crew attention. Consequently, it is quite beneficial to provide automatic control systems for relieving some of the pilot's many in-flight tasks. Autothrottles and electronic synchronizers, which manage the engine thrust by controlling the fuel supplied to the engines, are typically some of these automatic systems.
In multi-engine aircraft it is often desirable to have independent fuel controls over the several engines so as to enable the pilot or a flight control computer to effectively match the thrust and frequencies of the engines, thereby regulating the speed and attitude of the aircraft and also abating the ubiqitous beat frequencies which often perturb the passengers. Another concern in modern aircraft design, which reflects the current heightened attention to air travel safety, is to provide ultra-highly reliable mechanical systems throughout the aircraft.
One type of autothrottle system which has been frequently used in the past, basically provides a computer controlled servo unit connected to and mechanically manipulating the throttle control handles which are located at the pilot's side. The servo unit is usually connected to the throttle handles, as a unit, so that when the servo operates the handles track together, i.e., each is moved the same amount. These throttle control handles are typically connected to the engines by elongated cables, so that when the throttle control handles are moved, either by the pilot or by the servo, the cables and their throttle connections at the engine are also moved. This movement results in the regulation of fuel to the engines.
While this system, or variations or it, have been extensively used for automatically controlling the engine thrust, it does have numerous serious drawbacks. The throttle controls and the elongated cables are typically connected to the servo by a common shaft; this configuration does not enable automatically controlling the separate engine fuel supplies on an independent basis. Therefore, the fine tuning or matching of the engine speeds is not fulfilled by the autothrottle systems. Rather, it is either left as a pilot duty or is controlled by a separate and expensive on-board electronic engine control system. Furthermore, due to the conventional reluctance of design engineers to double the weight of an autothrottle system by adding a symmetrically redundant automatic back-up system. The duty to regulate engine thrust and engine fuel is typically returned to the pilot in the event of an autothrottle system failure. Moreover, typical system designs have an additional undesirable drawback; the elongated cables extending from the servo aft to the engines experience considerable stretching, thereby causing "play" or "dead zones" to appear in the throttle control system. These dead zones traditionally have caused limit cycles of several knots when the throttle controls aircraft speed.
During the protracted and expensive experimentation leading up to this invention, numerous designs were attempted, each of which was thought to be inferior in some regard. For example, in meeting the independent engine control requirement it was thought to provide a separate primary autothrottle system, i.e., a flight control computer system, servo and connecting cables etc., for each engine. It was then through that the "dead zones" could be reduced in such a system by positioning the servo units closer to the engine and thereby reducing the cable length and corresponding cable stretch. These solutions were unattractive when the system was also required to have a back-up or redundant system. If each engine has its own primary autothrottle and a redundant system to take over if a primary servo or computer failed, it was clear that the need for two servos and two computers for each engine may be uneconomical and also would likely create serious weight considerations in multi-engine aircraft, especially those with rear-mounted engines where the added weight might adversely change the critical position of the plane's center of gravity.
Consequently, a great need exists for improvement in autothrottle control and synchronizer systems which provide for independent fuel control for each of the several engines, a reduction in the "play" or "dead zones", and a redundant automatic back-up system without doubling the weight of the autothrottle and synchronizer systems.
It is an object of this invention to provide an autothrottle and synchronizer system which independently controls several engines of a multi-engine aircraft while concurrently providing a back-up system in the event of a primary system failure.
It is a feature of this invention to have a selectable cross-connection between the primary throttle control servos of two engines so that if either one of the primary servos or computers fails, the primary servo and computer of the other engine, together with the selectable cross-connection, will operate the throttle which was serviced by the failed component.
It is an advantage of this invention to enable a reduction in the cost and weight of a redundant autothrottle and synchronizer system in that the need for four servos, two computers and two electronic engine synchronizers to operate a complete primary and redundant system for two engines is reduced to two servos and two computers along with a selectable cross-connection and no electronic engine synchronizers.